Recently, monolayers of layered transition metal dichalcogenides (LTMDs), such as MX2 (where M=Mo or W and where X=S or Se), have been reported to exhibit significant spin-valley coupling and optoelectronic performances because of their unique structural symmetry and band structures. Monolayers in this class of materials offered a burgeoning field in fundamental physics, energy harvesting, electronics and optoelectronics. Most studies to date, however, are hindered by the great challenges of synthesizing and transferring high-quality LTMD monolayers. Hence, a feasible synthetic process to overcome these challenges would be advantageous.
Considerable efforts have been devoted to synthesize an MoS2 monolayer, including various kinds of exfoliations, physical vapor deposition, and chemical vapor deposition (CVD). Recently, a CVD-MoS2 monolayer was presented with sulfurization of the thin Mo layer and induced layer growth using fragments of reduced graphene oxide as seeds. Y. Zhan, et. al, “Large-area vapor-phase growth and characterization of MoS2 atomic layers on a SiO2 substrate,” Small, 8, 966-971 (2012). The as-grown layers, however, displayed obvious thickness variation; and their optoelectronic performance was a few orders of magnitudes worse than that of exfoliated layers. Further applications and scientific study have been hindered due to reduced mobility and a low on-off current ratio because of the high defect concentration and small grain size. Accordingly, most studies still use exfoliated samples since the synthesis of high-quality LTMD monolayers has remained a great challenge thus far.